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Fish oil supplements of so-called omega-3 fatty acids may not do much to ward off heart attacks and strokes in people who already have heart disease, according to an international analysis.

The research, which appeared in the Archives of Internal Medicine and covered 14 studies, found that there was no difference in the number of heart attacks, strokes or deaths among more than 20,000 people with heart disease who were randomly assigned to take either fish oil supplement or fish oil free placebo pills.

Research has been mixed on the possible heart-related benefits of the omega-3 fatty acids, specifically those known as EPA and DHA, which can be taken as fish oil supplements as well as eaten. Groups such as the American Heart Association recommend at least two servings of such fish a week.

"There is a common perception that fish oil supplements have been proven to prevent cardiovascular disease, and in fact the evidence has been inconsistent and inconclusive," says JoAnn Manson, head of preventive medicine at Brigham and Women's Hospital in Boston, who co-wrote a commentary published with the study.

"It's an important issue, because a large percentage of the population is taking fish oil supplements over-the-counter," she says.

Meta-analysis of studies

Researchers from Korea combined the results of 14 studies that tracked heart disease patients taking fish oil or a placebo, without knowing which they were getting, for between one and five years. That included reports from the United States and India, as well as Italy, Germany and elsewhere in Europe.

The patients were mostly male and in their mid-60s, on average.

Those who were assigned to take the fish oil supplements were just as likely to have a range of heart-related emergencies, or to die, as study participants taking placebos containing vegetable oil or other substances not associated with heart health.

For example, in one study from the Netherlands, 14 per cent of people in either group had a stroke or heart attack, or needed a stent implanted, over about three and a half years.

There were also no differences in deaths or other heart and blood vessel problems when the researchers looked specifically at people taking higher or lower doses of fish oil, or among those who took the supplements for only a year or two or for longer, says Seung-Kwon Myung of the National Cancer Center in Ilsan, and colleagues.

Myung says he doesn't recommend that people, either with or without a history of heart disease, take the supplement to prevent future problems.

Longer-term studies needed

"At this point, we don't have enough hard-and-fast data to suggest routine supplementation with fish oil," says Alice Lichtenstein, a nutritional scientist at Tufts University in Boston, who wasn't involved in the study.

She notes that the studies in the review tended to only cover a couple of years on the supplements, and a heart attack or stroke can take much longer to develop.

Longer-term studies are needed on the impact of fish oil, says Manson. But for now people should continue to follow recommendations that advise at least two servings a week of fatty fish.

"Supplements will never be a replacement for a healthy dietary pattern, because often the healthier food choices such as fish can take the place of less-healthy food such as red meat," she added.